There is no problem to change the NS records for your domain.

SiteGround customers can make this change from:

Customers' Area --> My Account --> Manage (next to the domain name you need the NS records updated for) > Your Domain DNS.

Click the Change button, modify the NS records for your domain and then click Update to save the changes. It will take several hours for the DNS change to propagate.

 
If you are not a SiteGround customer, you should be able to edit your NS records from the control panel at your host/registrar. For more information on how to do this you will have to contact your host/registrar.